Electrochemical gas sensors traditionally comprise a gas diffusion working or sensing electrode, often based on a metal catalyst dispersed on PTFE tape. A target gas reacts at this electrode while a balancing reaction takes place at the counter electrode, which may also be a gas diffusion electrode. The electrodes are held within an outer housing which usually contains a liquid electrolyte capable of supporting the relevant reactions (e.g., sulfuric acid). The gas under test typically enters the housing through a controlled diffusion access port which regulates the ingress of the target gas into the cell. As the target gas is reacted at the sensing electrode, the electrical output of the sensor may be directly related to the ambient target gas concentration. Such principles are well known and have been described.
There are a number of key performance parameters which limit the use of electrochemical gas sensors in aggressive environments. One of these is the ability of the sensor to function for extended periods in extremes of temperature and/or humidity. Traditional electrolytes are often based on aqueous systems which have particular weakness in this regard. Clearly it is desirable for the sensor's working lifetime to be as long as possible but moreover it is important that any particular sensor type will consistently continue to work for at least the indicated lifetime. Early failures lead to the need for more frequent sensor replacement, as well as increased checking and monitoring of sensor performance and, ultimately, a loss in confidence in the sensor. Accordingly, there is a need to produce sensors that are more stable under many different operating environments. There has been some progress in improving this behavior by using novel electrolyte systems such as those based on ionic liquids.
A further limitation of some current electrochemical gas sensors is that the available electrode options do not have the required level of specificity toward the target gas. This can require the use of filters to improve the selectivity, which can in turn introduce further operational difficulties.
Some electrode types are also relatively expensive to manufacture, usually due to the presence of comparatively high loadings of precious metal catalysts. Unfortunately, many conventional supported catalysts (which could be used as a cheaper alternative) are not stable in the aggressive electrolytes currently employed, which further limits the available options.
We have demonstrated that the behavior of electrode-electrolyte systems is affected in previously unanticipated ways by switching from conventional (aqueous) electrolytes to those based on ionic liquids. This has been shown to offer the sensor designer a range of alternative tools and options which can solve key operational problems.
In particular, we have shown that the same electrode material (e.g. Pt) used to detect a particular target gas (e.g. H2S) will demonstrate quite different relative sensitivities towards potential gas phase interferents, depending on the electrolyte system used. We have identified key differences in behavior between conventional and ionic liquid electrolytes, as well as between different types of ionic liquid electrolyte. This effect can have a major impact in practical applications, where conventional electrolytes may require other means (e.g. filters) to ensure that a reliably specific measurement of the target gas is obtained.
We have also noted that some electrode choices which might otherwise be commercially preferred are currently excluded by cross interference effects. Thus for H2S sensors using conventional H2SO4 based electrolyte, pure Pt is not a viable choice due to the magnitude of its CO cross interference. The usual solution is to employ Ru/Pt or Ir. However, these are more susceptible to poisoning, generally offer lower activity and are less amenable to conventional electrode manufacturing processes. Pt/carbon mixtures, which might offer a means to reduce the unwanted CO interference, cannot be employed because the carbon is prone to slow oxidation at anodic potentials in acid electrolyte media, giving rise to long term instability. However, this problem can be eliminated by the appropriate choice of ionic liquid electrolyte, where the different cross sensitivity behavior offered provides new options to the sensor designer. This approach enables resolution of some key practical operating problems.
A further example relates to oxygen pump sensors where oxygen is reduced at the working electrode and evolved at the counter. This system normally employs acidic electrolytes similar to those used in many toxic gas sensors and so the use of carbon-supported Pt catalysts for oxygen reduction is again unfavorable due to their poor stability in this environment. However, major cost savings can be obtained by the reduction of precious metal loading facilitated by the use of carbon-supported catalysts. This can in principle be facilitated by the use of an ionic liquid electrolyte, in which the carbon is much more stable.